Manchester Earthquakes Sequence, October - November 2002

A sequence of more than 100 earthquakes started in the
Greater Manchester area of the United Kingdom on October 19,
2002. Three temporary seismograph stations were installed to
supplement existing permanent stations and to better
understand the relationship between the seismicity and local
geology. Due to the urban location, the events were
experienced by a large number of people. The first
earthquake that was felt occurred on 21 October at 07:45:15
(UTC) with a magnitude 3.2 ML. A few hours later at
11:42:34, there was a magnitude 3.9 ML earthquake, the
largest in the sequence, which was strongly felt throughout
the Greater Manchester area. About 22 seconds later there
was another earthquake with a magnitude 3.5 ML. Numerous
smaller earthquakes occured until the last event on 30
November 2011.

Precise Relocation

Analysis of the events in the sequence showed that many of
them were very similar. Such similarity only occurs if the
earthquakes are closely located and have a similar fault
mechanism. We were then able to determine relative locations
for the earthquakes by measuring small differences in the
travel times of seismic waves between different events. The
new locations show a strong alignment along a
northwest-southeast trend (see map). Source mechanisms
determined for the largest of the earthquakes suggest either
northeast-southwest or northwest-southeast striking fault
planes. The northwest-southeast plane agrees with the
observed trend of the hypocentres, suggesting that this is
probably the orientation of the fault plane. Numerous faults
have been mapped at the surface, which mostly strike
north-south or northwest-southeast. Again, a
northwest-southeast strike agrees with both the strike of
the epicentres and the calculated focal mechanisms. The coal
measures in and around Manchester were worked extensively up
to the 1970s. However, the earthquakes occurred at a depth
of 6-7 km, which places them far below even the deepest mine
workings.

Earthquake Swarms

Earthquake sequences can occur in two ways. Firstly,
moderate to large size earthquakes are usually followed by
aftershocks, which occur due to readjustment to a new state
of stress. The duration of the aftershock sequence depends
partly on the size of the event. Normally, the largest
aftershock is about one magnitude unit smaller than the main
shock. For example, the magnitude 5.4 ML Lleyn Peninsula
earthquake that occurred in North Wales in 1984 was followed
by some hundreds of aftershocks in the subsequent months,
the largest of which was a magnitude 4.3 ML earthquake that
occurred one month later.

Secondly, earthquake swarms are sequences of earthquakes
clustered in time and space without a clear distinction of
main shock and aftershocks. Such earthquake sequences or
swarms are relatively common in Great Britain, examples
include Comrie (1788-1801, 1839-46), Kintail (1974),
Blackford (1997-98, 2000-01), Constantine (1981, 1986,
1992-4), Johnstonebridge (mid 1980s) and Dumfries (1991,
1999). The largest event in the Comrie sequence was a
magnitude 4.8 ML earthquake in 1839.

Felt effects

The BGS received more than 3000 reports frmo people who felt
the Manchester earthquakes. Most of the reports were from
within 20 km from the Manchester city centre. Reports of
minor damage, such as small cracks, falling tiles, shattered
windows, falling rubble and plaster were also received. A
maximum intensity of 5+ on the European Macroseismic Scale
(EMS) was determined for the earthquake on October 21 at
11:42.

Epicentres of the Manchester earthquakes. Blue squares show single event locations. Red squares show revised locations using the precise relative arrival times and double difference method. CLICK TO ENLARGE

Seismograms for a number of similar events recorded on the seismometer at Manchester University, which was closest to the epicentre. CLICK TO ENLARGE

Source mechanisms for six of the
largest earthquakes in the Manchester sequence. The solution
was obtained by using first motion polarities and amplitude
ratios. CLICK TO ENLARGE

Simplified geology in the area of the earthquakes, taken from the BGS 1:50,000 geological map. Also shown are the relocated epicentres (blue squares). CLICK TO ENLARGE.